Method for inhibiting calcium salt scale

ABSTRACT

Compositions and method of improving inhibition of calcium salt scale formation under the conditions found in chemical pulp processes in which an effective amount of selected phosphonates or phosphonate blends is admixed with the black liquor composition recovered from the digester in a chemical pulping process. The compositions and method are especially well suited for use in the Kraft pulping process.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a nonprovisional application which claims thepriority of prior provisional application serial No. 60/296,356,entitled “Method for Inhibiting Calcium Salt Scale,” filed Jun. 6, 2001,which is hereby incorporated by reference into this application.

FIELD OF THE INVENTION

[0002] This invention relates to compositions and methods for inhibitingscale formation in aqueous alkaline systems of chemical pulpingprocesses. This invention further relates to compositions and methodsfor inhibiting formation, deposition and adherence of calcium salt scaledeposits in chemical pulping process equipment. More particularly, thisinventions relates to compositions and methods for inhibiting formation,deposition and adherence of calcium salt scale deposits in the blackliquor recovery area of a chemical pulping process.

BACKGROUND OF THE INVENTION

[0003] Paper is widely used worldwide in commerce and in homes and has avariety of uses. Pulp making is thus carried out on a large industrialscale worldwide to produce sufficient quantities of paper. Accordinglyit is highly desirable that such pulp making operations be carried outin a cost effective, efficient operation with minimum manufacturingequipment downtime and minimum periods of reduced pulp making processequipment efficiency.

[0004] The basic steps in industrial pulp making are to convert plantfiber into chips, convert chips into pulp, (optionally) bleach the pulp,wash the pulp, and transform the pulp into suitable paper which can beused in paper products such as writing paper, newsprint and paper fordocuments.

[0005] Typically, several chemical pulping processes are used inindustrial pulp making operations. Well known industrial alkalinechemical pulping processes include the Kraft (or sulfate), soda andalkaline sulfite processes. The Kraft process makes the strongest fibersof any pulp producing process and is the most commonly used pulp makingprocess in part due to its efficient recovery process for the cookingchemicals. While the present invention has applicability to any of theabove alkaline chemical pulping processes, it is particularly usefulwith the Kraft process and, as such, the Kraft process is described inmore detail below.

[0006] Initially, suitable trees are harvested, debarked and thenchipped into suitable size flakes or chips. These wood chips are sortedwith the small and the large chips being removed. The remaining suitablewood chips are then charged to a digester (which is a vessel or tank forholding the chips and an aqueous digesting composition, such tanks canbe designed for either batch or continuous operation).

[0007] Illustratively, in a batch type digester, wood chips and amixture of “weak black liquor,” the spent liquor from a previousdigester cook, and “white liquor,” a solution of sodium hydroxide andsodium sulfide, that is either fresh or from the chemical recoveryplant, is pumped into the digester. In the cooking process lignin, whichbinds the wood fiber together, is dissolved in the white liquor formingpulp and black liquor.

[0008] The digester is sealed and the digester composition is heated toa suitable cook temperature under high pressure. After an allottedcooking time at a particular temperature and pressure (H-factor) in thedigester, the digester contents (pulp and black liquor) are transferredto a holding tank. The pulp in the holding tank is transferred to brownstock washers while the liquid (black liquor formed in the digester) issent to the black liquor recovery area, i.e. black liquor evaporators.The black liquor is evaporated to a high solids content, usually 60-80%solids, using a multiple effect evaporator, for example. The higher thesolids content, the more difficult it is to pump the black liquor andthe more scale problems the pulp mill will have. One of the mosttroublesome is calcium carbonate scale which forms in various areas ofthe pulp mill, including the digester, the black liquor evaporator area,and the brown stock washing area.

[0009] Most commercial paper mills use multiple effect evaporators (MEE)as the black liquor evaporators. These evaporators generally range fromfour to eight effects in length. Generally, undesirable calciumcarbonate scaling occurs in only one or two effects. Currently, mostmills do not use any scale inhibitor but rather contend with the scaleproblem by shutting down the black liquor evaporator section and washingout the calcium carbonate scale with hot acid, i.e. acid cleaning. Thishot acid boil out adversely affects papermill production and is aconcern because the acid used is corrosive to mill piping and equipment.

[0010] The Kraft cook is highly alkaline, usually having a pH of 10 to14, more particularly 12 to 14. The digester composition contains alarge amount of sodium sulfide, which is used as an accelerant toincrease the delignification rate of the cook. This works to release thelignin in the wood chips and thus the cellulose becomes available aspulp.

[0011] The combination of operating conditions in the Kraft process isconducive to scale formation and deposition and increases the propensityof the calcium carbonate scale to form, deposit and adhere to metallicand other surfaces within which it comes in contact. Under such processconditions, calcium present in the water and leached from the wood inthe Kraft process can react with carbonate and produce rather rapidscaling with the deposition of calcium carbonate scale. Such scale isfrequently deposited in the black liquor evaporator, the digester, andassociated piping, heat exchangers, etc., all of which have surfaces onwhich the calcium carbonate can deposit and adhere. Such depositionbuilds up over time and can result in undesirable premature shutdownsdownstream on the pulp making manufacturing line to remove scaledeposits by hot acid washing.

[0012] Several patents and a technical article disclose problems ofscaling. In “An Effective Sequestrant For Use In Controlling DigesterScale,” R. H. Windhager, Paper Trade Journal, pp. 42-44, Nov. 5, 1973,the use of small quantities of mono-aminomethylene phosphonic acid(ATMP) as a calcium carbonate scale inhibitor in a digester to inhibitscale deposition from the digester cooking liquor is disclosed.

[0013] U.S. Pat. No. 4,799,995 (issued to Druce K. Crump et al. on Jan.24, 1989) discloses that inhibition of calcium scale under conditionsfound in pulp digesters has been accomplished by employing mixtures ofpolyamino(polyalkylenephosphonic) acids with non-ionic surfactants addedto the pulp liquor. This U.S. patent also discloses that phosphonatessuch as nitrilotris(methylenephosphonic acid) (“NTMP” or “ATMP”),1-hydroxyethane-1,1-diphosphonic acid (“HEDP”) and sodium1-hydroxyethane-1,1-diphosphonate (“NaHEDP”) are said to have beencommonly used to control scale. However, the '995 patent discloses thatthe use of HEDP in black liquor actually promoted scale and use ofdiethylenetriamine penta(methylenephosphonic acid) (“DTPMP”) in blackliquor without the presence of a nonionic surfactant resulted in onlylimited scale reduction. While the '995 patent discloses the use ofnonionic surfactants to improve scale reduction, it is preferred toavoid the use of surfactants in chemical pulp processes, particularly inthe digester. The compositions of the present invention when added to analkaline chemical pulp process digester are effective at inhibitingcalcium salt scale in chemical pulp processes without the need for anonionic surfactant.

[0014] Canadian Patent No. 1,069,800 (Philip S. Davis et al., Jan. 15,1980) discloses the addition of blends of organophosphonates, e.g.1-hydroxyethylidene-1,1-diphosphonic acid (HEDP), with amino-organophosphonates, e.g. amino tri(methylenephosphonic acid) (AMP),ethylenediamine tetra(methylenephosphonic acid) (EDTPA), andhexamethylenediamine tetra(methylenephosphonic acid) (HMDTA), to blackliquor to reduce calcium carbonate scale in a black liquor evaporatorsystem at a pH above 9. This patent also discloses that use ofindividual (single) phosphonates, instead of the disclosed blends, werenot effective at a pH above 9 to inhibit calcium carbonatecrystallization.

[0015] U.S. Pat. No. 4,851,490 (issued to Fu Chen et al. on Jul. 25,1989) discloses water soluble polymers containinghydroxyalkyleneaminoalkylene phosphonate functions which are said tohave utility as deposit control agents effective in a number of watersystems such as cooling, boilers, conversion coating, paper and pulpprocessing and gas scrubbing.

[0016] U.S. Pat. No. 5,534,157 (issued to Craig D. Iman et al. on Jul.9, 1996) discloses a method for inhibiting the formation, deposition andadherency of scale-forming salts in process waters at high pH utilizingpolyether polyamine methylene phosphonates. At column 4, lines 35-51thereof, this U.S. patent discloses that inhibitors such as HEDP andATMP are useless as scale inhibitors at alkaline pH conditions.

[0017] U.S. Pat. No. 5,562,830 (issued to Davor F. Zidovec et al. onOct. 8, 1996) discloses a method of inhibiting corrosion and scaleformation and deposition in aqueous systems by adding a combination of apolyepoxysuccinic acid or salts thereof and a phosphonocarboxylic acidor salts thereof.

[0018] U.S. Pat. No. 5,552,018 (issued to Johan Devenyns on Sep. 3,1996) discloses a process in which a peroxyacid is employed to improvethe selectivity of the delignification of a chemical paper pulp that hasalready undergone a delignifying treatment in the presence of chemicalreagents, i.e. a Kraft cook. Phosphonates are disclosed as stabilizersin this process.

[0019] Despite the aforementioned patents and technical article,enhanced methods and compositions for inhibiting the formation,deposition and adherence of scale to metallic surfaces particularly incommercial chemical pulp processing equipment is highly desired.

SUMMARY OF THE INVENTION

[0020] It is an object of this invention to provide a composition forinhibiting the formation, deposition and adherence of calcium salt scaleto metallic and other surfaces in the equipment, vessels and/or pipingof a chemical pulp process facility. It is another object of thisinvention to provide a method for inhibiting the formation, depositionand adherence of calcium salt scale to surfaces in the equipment,vessels and/or piping of a chemical pulp process facility.

[0021] These and other objects are achieved in the invention which isdescribed in more nonlimiting detail hereinafter.

[0022] According to the invention, a scale inhibiting composition forinhibiting calcium salt scale formation in alkaline aqueous mixtures ofchemical pulping processes is provided, wherein the composition is addedto the black liquor of the chemical pulping process, the compositioncomprising an effective scale inhibiting amount of at least onephosphonate selected from compounds having the formula:

M₂O₃P—CH₂—N(R¹)—(CH₂)_(m)—N(R²)—CH₂PO₃M₂  (I),

[0023] compounds having the formula:

R³—C(OH)(PO₃M₂)₂  (II),

[0024] compounds having the formula:

N—(CH₂PO₃M₂)₃  (III),

[0025] phosphonates having the formula:

[0026] amine oxides of phosphonates of formulas (I) and (III), ormixtures thereof; wherein M is independently selected from hydrogen,alkali metal, alkaline earth metal or ammonium, R¹ and R² areindependently selected from CH₂PO₃M₂ or (CH₂)_(n)—N—(CH₂PO₃M₂)₂, m is 2or 3, n is 2 or 3, and R³ is an alkyl group having 1 to 17 carbon atomsand R³ is optionally branched and optionally unsaturated; with theprovisos that:

[0027] (a) the phosphonate is not a blend of a phosphonate of formula(II) with a phosphonate of formula (III),

[0028] (b) the phosphonate is not a blend of a phosphonate of formula(II) with a phosphonate of the formula(M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)₂,

[0029] (c) when the phosphonate is selected from phosphonates of formula(III), phosphonates of the formula (M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)₂,or phosphonates of the formula(M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)—(CH₂)₂—N—(CH₂PO₃M₂)₂, the scaleinhibiting composition does not contain a nonionic surfactant,

[0030] (d) when the phosphonate is selected from phosphonates of formula(III), or phosphonates of the formula(M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)—(CH₂)₂—N—(CH₂PO₃M₂)₂, the amount ofthe phosphonate on an active acid basis is greater than 25 ppm based onthe weight of black liquor recovered from the digester, and

[0031] (e) when the phosphonate is selected from the phosphonates offormula (IV), the amount of the phosphonate on an active acid basis isgreater than 20 ppm based on the weight of black liquor recovered fromthe digester.

[0032] Further according to the invention, a method for inhibitingcalcium salt scale formation in chemical pulping processes is providedcomprising admixing an effective scale inhibiting amount of the abovecomposition with the black liquor recovered from the digester of thechemical pulping process.

[0033] Still further according to the invention, a method for inhibitingcalcium salt scale formation in an aqueous system in a selected alkalinechemical pulping process is provided comprising: (a) determining thecalcium salt scale inhibition profiles of phosphonate concentration andprocess temperature as a function of time for phosphonate compositionsadmixed with the black liquor composition recovered from the digester ofthe chemical pulping process, (b) identifying the calcium salt scaleinhibition capability required by the selected chemical pulping processbased on the process operating conditions of time, temperature andpressure, and the black liquor composition, (c) selecting theappropriate phosphonate composition and phosphonate use concentration toeffectively inhibit calcium salt scale formation in the selectedchemical pulping process when the phosphonate is admixed with the blackliquor composition recovered from the digester of the selected alkalinechemical pulping process based on steps (a) and (b), and (d) admixingthe selected phosphonate composition with the black liquor compositionin the selected alkaline chemical pulping process during the blackliquor recovery stage of the chemical pulping process; wherein theselected phosphonate composition is as defined above.

[0034] Still further according to the invention, a method for inhibitingcalcium salt scale formation in an aqueous system in a selected alkalinechemical pulping process is provided comprising: (a) identifying thecalcium salt scale inhibition capability required by the selectedchemical pulping process based on the process operating conditions oftime, temperature and pressure, and the black liquor composition, (b)selecting the appropriate phosphonate composition and phosphonate useconcentration to effectively inhibit calcium salt scale formation in theselected alkaline chemical pulping process when the phosphonate isadmixed with the black liquor composition recovered from the digester inthe selected alkaline chemical pulping process based on step (a) and thecalcium salt scale inhibition profiles of phosphonate concentration andprocess temperature as a function of time for phosphonate compositionsadmixed with the black liquor composition recovered from the digester ina chemical pulping process, and (c) admixing the selected phosphonatecomposition with the black liquor composition recovered from thedigester in the selected alkaline chemical pulping process during thedigestion stage of the chemical pulping process; wherein the selectedphosphonate composition is as defined above.

DETAILED DESCRIPTION OF THE DRAWINGS NOT APPLICABLE DETAILED DESCRIPTIONOF THE INVENTION

[0035] A first embodiment of the invention relates to a scale inhibitingcomposition for inhibiting calcium salt scale formation in alkalineaqueous mixtures of chemical pulping processes, wherein the compositionis added to the black liquor of the chemical pulping process, thecomposition comprising an effective scale inhibiting amount of at leastone phosphonate selected from compounds having the formula:

M₂O₃P—CH₂—N(R¹)—(CH₂)_(m)—N(R²)—CH₂PO₃M₂  (I),

[0036] compounds having the formula:

R³—C(OH)(PO₃M₂)₂  (II),

[0037] compounds having the formula:

N—(CH₂PO₃M₂)₃  (III),

[0038] phosphonates having the formula:

[0039] amine oxides of phosphonates of formulas (I) and (III), ormixtures thereof; wherein M is independently selected from hydrogen,alkali metal, alkaline earth metal or ammonium, R¹ and R² can be thesame or different and are independently selected from —CH₂PO₃M₂ or—(CH₂)_(n)—N—(CH₂PO₃M₂)₂′, m is 2 or 3, n is 2 or 3, and R³ is an alkylgroup having 1 to 17 carbon atoms and R³ is optionally branched andoptionally unsaturated; with the provisos that:

[0040] (a) the phosphonate is not a blend of a phosphonate of formula(II) with a phosphonate of formula (III),

[0041] (b) the phosphonate is not a blend of a phosphonate of formula(II) with a phosphonate of the formula(M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)₂,

[0042] (c) when the phosphonate is selected from phosphonates of formula(III), phosphonates of the formula (M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)₂,or phosphonates of the formula(M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)—(CH₂)₂—N—(CH₂PO₃M₂)₂, the scaleinhibiting composition does not contain a nonionic surfactant,

[0043] (d) when the phosphonate is selected from phosphonates of formula(III), or phosphonates of the formula(M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)—(CH₂)₂—N—(CH₂PO₃M₂)₂, the amount ofthe phosphonate on an active acid basis is greater than 25 ppm based onthe weight of black liquor recovered from the digester, and

[0044] (e) when the phosphonate is selected from the phosphonates offormula (IV), the amount of the phosphonate on an active acid basis isgreater than 20 ppm based on the weight of black liquor recovered fromthe digester.

[0045] In the phosphonates of the invention, M is preferably hydrogen oralkali metal, and the alkali metal is preferably sodium or potassium, R³is preferably an alkyl group having 1 to 5 carbon atoms, more preferablymethyl, and m is preferably 2.

[0046] The scale inhibiting compositions of the invention include, butare not limited to, at least one phosphonate of formula (I), at leastone phosphonate of formula (II), at least one phosphonate of formula(III), at least one phosphonate of formula (IV), at least one amineoxide of a phosphonate of formulas (I) or (III), a mixture of at leasttwo phosphonates of formula (I), a mixture of at least one phosphonateof formula (I) and at least one phosphonate of formula (II), formula(III) or formula (IV), a mixture of at least two phosphonates of formula(II), or a mixture of at least one amine oxide of a phosphonate offormulas (I) or (III) and at least one phosphonate of formulas (I) or(III). Preferably, the scale inhibiting composition of the invention isat least one phosphonate of formula (I), a mixture of at least twophosphonates of formula (I), a mixture of at least one phosphonate offormula (I) and at least one phosphonate of formula (II) or formula(III), or a mixture of at least one phosphonate of formula (II) and atleast one phosphonate of formula (IV).

[0047] Examples of suitable phosphonates include, but are not limitedto, the phosphonates in Table 1 below. Table 1 below provides formulasfor representative phosphonates of formulas (I), (II) and (III). Thephosphonates in Table 1 are available from Solutia Inc., 575 MaryvilleCentre Drive, St. Louis, Mo. under the trademark Dequest® phosphonatesand are identified by their Dequest® phosphonate product number. TABLE 1Dequest Product No. Formula R¹ R² m n R³ M 2000 III — — — — — 6 H 2006III — — — — — 5 Na, 1 H 2010 II — — — — CH₃ 4 H 2016 II — — — — CH₃ 4 Na2041 I CH₂PO₃M₂ CH₂PO₃M₂ 2 — — 8 H 2046 I CH₂PO₃M₂ CH₂PO₃M₂ 2 — — 3H, 5Na 2060 I CH₂PO₃M₂ (CH₂)_(n)N(CH₂PO₃M₂)₂ 2 2 — 10 H 2066 I CH₂PO₃M₂(CH₂)_(n)N(CH₂PO₃M₂)₂ 2 2 — 7 Na, 3 H 7000 IV — — — — — 5 H

[0048] The formulas and corresponding names of the Dequest phosphonateslisted in Table 1 are shown below.

[0049] Dequest 2000—amino-tris(methylenephosphonic acid) N(CH₂PO₃H₂)₃

[0050] Dequest 2006—sodium salt of amino-tris(methylenephosphonic acid)Na₅H[N(CH₂PO₃)₃]

[0051] Dequest 2010—1-hydroxyethylidene (1,1-diphosphonic acid)CH₃C(OH)(PO₃H₂)₂

[0052] Dequest 2016—sodium salt of 1-hydroxyethylidene (1,1-diphosphonicacid) Na₄[CH₃C(OH)(PO₃)₂]

[0053] Dequest 2041—ethylenediamine tetra(methylenephosphonic acid)H₈[(O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃)₂]

[0054] Dequest 2046—ethylenediamine tetra(methylenephosphonic acid),pentasodium salt Na₅H₃[(O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃)₂]

[0055] Dequest 2060—diethylenetriamine-penta(methylenephosphonic acid)(H₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃H₂)CH₂CH₂N(CH₂PO₃H₂)₂

[0056] Dequest 2066—sodium salt ofdiethylenetriamine-penta(methylenephosphonic acid)Na₇H₃[(O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃)CH₂CH₂N(CH₂PO₃)₂]

[0057] Dequest 7000—2-phosphonobutane-1,2,4-tricarboxylic acid

[0058] Another preferred phosphonate of formula (I) is the compoundN,N′-bis(3-aminopropyl)ethylenediamine-hexa(methylenephosphonic acid),or a salt thereof wherein the salt is sodium, potassium, ammonium, andthe like. When the compound is the sodium salt, the compound has theformula

[0059]Na_(x)H_(y)[(O₃PCH₂)₂NCH₂CH₂CH₂N(CH₂PO₃)CH₂CH₂N(CH₂PO₃)CH₂CH₂CH₂N—(CH₂PO₃)₂];wherein x+y is 12, and is designated herein as 4NHMP. This compound canbe prepared according to the procedure disclosed in Example 1 of U.S.Pat. No. 5,261,491, which is herein incorporated by reference.

[0060] The preferred phosphonates of formula (I) are(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂,(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂, or(M₂O₃PCH₂)₂NCH₂CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂CH₂N—(CH₂PO₃M₂)₂,more preferably (M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂ or(M₂O₃PCH₂)₂NCH₂CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂CH₂N—(CH₂PO₃M₂)₂,and most preferably(M₂O₃PCH₂)₂NCH₂CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂CH₂N—(CH₂PO₃M₂)₂.

[0061] The preferred phosphonate of formula (II) is H₃C—C(OH) (PO₃M₂)₂,with H₃C—C(OH) (PO₃Na₂)₂being more preferred.

[0062] Blends of at least two phosphonates independently selected fromthe phosphonates of formulas (I), (II), (III), (IV) and amine oxides ofthe phosphonates of formulas (I) and (III) may be used according to theinvention. It is currently preferred to use a blend of two phosphonates,with a blend of a phosphonate of formula (I) with either a phosphonateof formula (I), formula (II), formula (III) or formula (IV) being morepreferred, and a blend of two phosphonates of formula (I) being mostpreferred. The composition of the blends can vary over a wide range withthe percentage of each component ranging broadly from 1 to 99 wt. %,provided each phosphonate is present in an amount of at least about 1wt. %. Preferably, each phosphonate is present in an amount of at leastabout 10 wt. %. In the case of a two component blend, each phosphonateis present preferably in an amount of about 10 to about 90 wt. %, andmore preferably in an amount of about 20 to about 80 wt. %.

[0063] The preferred blends for use in the invention are blends of aphosphonate selected fromN,N′-bis(3-aminopropyl)ethylenediamine-hexa(methylenephosphonic acid),diethylenetriamine-penta(methylenephosphonic acid), or salts thereofwith a phosphonate selected from the phosphonates of formulas (I), (II),(III) or (IV), or a phosphonate selected from ethylenediaminetetra(methylenephosphonic acid), or salts thereof with a phosphonateselected from the phosphonates of formulas (I) or (III). More preferredare blends of phosphonates selected fromN,N′-bis(3-aminopropyl)ethylenediamine-hexa(methylenephosphonic acid),ethylenediamine tetra(methylenephosphonic acid),diethylenetriamine-penta(methylenephosphonic acid), or salts thereofwith another phosphonate selected from the phosphonates of formula (I)and blends ofN,N′-bis(3-aminopropyl)ethylenediamine-hexa(methylenephosphonic acid),diethylenetriamine-penta(methylenephosphonic acid) or salts thereof witha phosphonate selected from the phosphonates of formula (II).

[0064] An effective amount of phosphonate or mixtures of phosphonates isemployed in making and using the scale inhibiting composition of thisinvention. That effective amount depends on the particularphosphonate(s) employed in practicing this invention and other factorsincluding, but not limited to, the digester composition, the operatingconditions (i.e. H-factor) of the digester, the black liquorcomposition, and operating conditions in the brown stock washing areaand black liquor recovery area, as well as other factors and conditionsknown to those of ordinary skill in the art. Selection of the effectiveamount of phosphonate will be readily apparent to one of ordinary skillin the art after reading this specification.

[0065] When the scale inhibiting composition of the invention is atleast one phosphonate of formula (I), the phosphonate(s) and theeffective scale inhibiting amount of each is as follows.

[0066] As used herein, the ppm usage level of scale inhibitor is basedon the weight of total liquor charged with the liquor assumed to have adensity of 1 g/mL.

[0067] When the phosphonate is (M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂, theeffective amount of the phosphonate on an active acid basis is about 10ppm to about 1000 ppm, preferably about 20 ppm to about 500 ppm, andmore preferably about 30 to about 500 ppm, based on the weight of blackliquor recovered from the digester.

[0068] When the phosphonate is(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂, the effective amount ofphosphonate on an active acid basis is about 30 ppm to about 1000 ppm,preferably about 40 ppm to about 500 ppm, based on the weight of blackliquor recovered from the digester.

[0069] When the phosphonate is(M₂O₃PCH₂)₂NCH₂CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂CH₂N—(CH₂PO₃M₂)₂,the effective amount of phosphonate on an active acid basis is about 10ppm to about 1000 ppm, preferably about 20 ppm to about 500 ppm, basedon the weight of black liquor recovered from the digester.

[0070] When the scale inhibiting composition of the invention is atleast one phosphonate of formula (II), the phosphonate is preferablyCH₃C(OH)(PO₃M₂)₂ and the effective scale inhibiting amount ofphosphonate on an active acid basis is about 20 ppm to about 200 ppm,preferably about 30 ppm to about 100 ppm, based on the weight of blackliquor recovered from the digester.

[0071] When the scale inhibiting composition of the invention is atleast one phosphonate of formula (III), the effective scale inhibitingamount of phosphonate on an active acid basis is about 50 to about 1000ppm, preferably about 80 to about 500 ppm, based on the weight of blackliquor recovered from the digester.

[0072] When the scale inhibiting composition of the invention is atleast one phosphonate of formula (IV), the effective scale inhibitingamount of phosphonate on an active acid basis is about 50 to about 500ppm, preferably about 100 to about 200 ppm, based on the weight of blackliquor recovered from the digester.

[0073] When the scale inhibiting composition of the invention is atleast one amine oxide of a phosphonate of formula (I) or formula (III),the effective scale inhibiting amount of amine oxide is the amount on anactive acid basis that is equivalent to the effective amount of thecorresponding phosphonate of formula (I) or formula (III).

[0074] When the scale inhibiting composition of the invention is amixture of at least two phosphonates of formula (I) of a mixture of atleast one phosphonate of formula (I) and at least one phosphonate offormula (III), the phosphonate(s) and the effective scale inhibitingamount of each is as follows.

[0075] When the first phosphonate is(M₂O₃PCH₂)₂NCH₂CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂CH₂—N(CH₂PO₃M₂)₂,the second phosphonate is preferably selected from N(CH₂PO₃M₂)₃,(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂, or(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂. When the secondphosphonate is N(CH₂PO₃M₂)₃, the amount of the mixture on an active acidbasis is about 10 ppm to about 1000 ppm, preferably about 200 ppm toabout 500 ppm, based on the weight of black liquor recovered from thedigester. When the second phosphonate is (M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂,the amount of the mixture on an active acid basis is about 20 ppm toabout 1000 ppm, preferably about 30 ppm to about 500 ppm, based on theweight of black liquor recovered from the digester. When the secondphosphonate is (M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂, theamount of the mixture on an active acid basis is about 10 ppm to about1000 ppm, preferably about 30 ppm to about 500 ppm, based on the weightof black liquor recovered from the digester.

[0076] When the first phosphonate is (M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂, thesecond phosphonate is preferably selected from(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂ or N(CH₂PO₃M₂)₃. Whenthe second phosphonate is(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂, the amount of themixture on an active acid basis is about 20 ppm to about 1000 ppm,preferably about 40 ppm to about 500 ppm, based on the weight of blackliquor recovered from the digester. When the second phosphonate isN(CH₂PO₃M₂)₃, the amount of the mixture on an active acid basis is about30 ppm to about 1000 ppm, preferably about 50 ppm to about 500 ppm,based on the weight of black liquor recovered from the digester.

[0077] When the first phosphonate is(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂, and the secondphosphonate is N(CH₂PO₃M₂)₃, the amount of the mixture on an active acidbasis is about 50 ppm to about 1000 ppm, preferably about 150 ppm toabout 500 ppm, based on the weight of black liquor recovered from thedigester.

[0078] The preferred blends of at least two phosphonates of formula (I)or at least one phosphonate of formula (I) and at least one phosphonateof formula (III) are blends of(M₂O₃PCH₂)₂NCH₂CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂CH₂—N(CH₂PO₃M₂)₂with N(CH₂PO₃M₂)₃, (M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂, or(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂, or blends of(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂ with(M₂O₃PCH₂)₂NCH₂CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂CH₂N—(CH₂PO₃M₂)₂,(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂, or N(CH₂PO₃M₂)₃.

[0079] The most preferred blends of at least two phosphonates of formula(I) or at least one phosphonate of formula (I) and at least onephosphonate of formula (III) are blends of(M₂O₃PCH₂)₂NCH₂CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)—CH₂CH₂CH₂N(CH₂PO₃M₂)₂with N(CH₂PO₃M₂)₃, (M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂, or(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂.

[0080] When the scale inhibiting composition of the invention is amixture of at least one phosphonate of formula (I) and at least onephosphonate of formula (II), the phosphonate(s) and the effective scaleinhibiting amount of each is as follows.

[0081] Preferred blends are mixtures of a first phosphonate selectedfrom(M₂O₃PCH₂)₂NCH₂CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂CH₂N—(CH₂PO₃M₂)₂or (M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂, and a secondphosphonate selected from CH₃C(OH)(PO₃M₂)₂.

[0082] When the first phosphonate is selected from(M₂O₃PCH₂)₂NCH₂CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂CH₂N—(CH₂PO₃M₂)₂,the amount of the mixture on an active acid basis is about 10 ppm toabout 500 ppm, preferably about 30 ppm to about 150 ppm, based on theweight of black liquor recovered from the digester. When the firstphosphonate is (M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂, theamount of the mixture on an active acid basis is about 30 ppm to about1000 ppm, preferably about 50 ppm to about 200 ppm, based on the weightof black liquor recovered from the digester.

[0083] The most preferred blends of at least one phosphonate of formula(I) and at least one phosphonate of formula (II) are blends of(M₂O₃PCH₂)₂NCH₂CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂CH₂N—(CH₂PO₃M₂)₂and CH₃C(OH)(PO₃M₂)₂.

[0084] A second embodiment of the invention relates to a method forinhibiting calcium salt scale formation in alkaline chemical pulpingprocesses comprising adding an effective scale inhibiting amount of atleast one phosphonate to the black liquor of the chemical pulpingprocess, the composition comprising at least one phosphonate selectedfrom compounds having the formula:

M₂O₃P—CH₂—N(R¹)—(CH₂)_(m)—N(R²)—CH₂PO₃M₂  (I),

[0085] compounds having the formula:

R³—C(OH)(PO₃M₂)₂  (II),

[0086] compounds having the formula:

N—(CH₂PO₃M₂)₃  (III),

[0087] phosphonates having the formula:

[0088] amine oxides of phosphonates of formulas (I) and (III), ormixtures thereof; wherein M is independently selected from hydrogen,alkali metal, alkaline earth metal or ammonium, R¹ and R² areindependently selected from —CH₂PO₃M₂ or —(CH₂)_(n)—N—(CH₂PO₃M₂)₂, m is2 or 3, n is 2 or 3, and R³ is an alkyl group having 1 to 17 carbonatoms and R³ is optionally branched and optionally unsaturated; with theprovisos that:

[0089] (a) the phosphonate is not a blend of a phosphonate of formula(II) with a phosphonate of formula (III),

[0090] (b) the phosphonate is not a blend of a phosphonate of formula(II) with a phosphonate of the formula(M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)₂,

[0091] (c) when the phosphonate is selected from phosphonates of formula(III), phosphonates of the formula (M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)₂,or phosphonates of the formula(M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)—(CH₂)₂—N—(CH₂PO₃M₂)₂, the scaleinhibiting composition does not contain a nonionic surfactant,

[0092] (d) when the phosphonate is selected from phosphonates of formula(III), or phosphonates of the formula(M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)—(CH₂)₂—N—(CH₂PO₃M₂)₂, the amount ofthe phosphonate on an active acid basis is greater than 25 ppm based onthe weight of black liquor recovered from the digester, and

[0093] (e) when the phosphonate is selected from the phosphonates offormula (IV), the amount of the phosphonate on an active acid basis isgreater than 20 ppm based on the weight of black liquor recovered fromthe digester.

[0094] In the practice of the method of this invention in a chemicalpulping process, e.g. a Kraft process, the aqueous phosphonatecomposition of the invention is admixed with the black liquor recoveredfrom the digester. The aqueous phosphonate composition of the inventioncan be added to the black liquor using any conventional means known tothose of ordinary skill in the art. In addition, the aqueous phosphonatecomposition of the invention can be added directly to the black liquorprior to the black liquor recovery stage, i.e. prior to the black liquorevaporator, or it can be added to the black liquor during the blackliquor recovery stage, e.g. between effects of the MEE. A typicaltemperature range in the black liquor evaporator is generally in therange of about 80 to about 180° C., depending on the effect. The pH ofthe black liquor in an alkaline chemical pulping process is at least 9.In the case of a Kraft process, the pH of the black liquor is typicallyabout 10 to about 14, and more typically about 12 to about 14. Theaqueous phosphonate composition of the invention can be added to theblack liquor in any conventional manner known to one of ordinary skillin the art. For example, in a batch digester operation, the addition ofthe aqueous phosphonate composition of the invention can be a bulkaddition at the beginning of the black liquor recovery stage or duringthe black liquor evaporator cycle, or it can be added in multiplecharges throughout the black liquor evaporator cycle, or continuously asthe black liquor is recovered. It is currently preferred to add theaqueous phosphonate composition of the invention as a bulk charge at ornear the beginning of the black liquor evaporator cycle. In the case ofa continuous digester operation, the aqueous phosphonate composition ofthe invention can be added continuously to the black liquor to maintainthe effective concentration of phosphonate in the black liquor or, ifthe black liquor is held in a storage vessel prior to the black liquorevaporator stage, it can be added as described above.

[0095] The amount of a scale inhibiting composition of this inventionemployed is an effective amount which is that amount that is sufficientto provide an effective scale inhibiting concentration of phosphonate inthe black liquor evaporator over time at which the formation, depositionand adherence of calcium salt scale, particularly calcium carbonate orcalcium sulfate scale, is satisfactorily inhibited in the black liquorrecovery area, and additionally in the digester and/or brown stockwashers. One of ordinary skill in the art using this invention will knowthe acceptable level of calcium salt scale in the digester, brown stockwashing area, and black liquor recovery area of the particular chemicalpulping facility, and will be able to readily select an appropriatephosphonate and concentration for addition to the black liquor toachieve the desired scale inhibition for the required time based on thedisclosure of this specification. It will be apparent to those of skillin the art after reading this specification that many factors of thetype which have been mentioned herein and others, will determine theamount of the phosphonate of the invention needed to achieve the desiredinhibition. The determination of these amounts is within the ordinaryskill of the artisan in this field without undue experimentationconsidering the direction provided herein.

[0096] A third embodiment of the invention relates to a method forinhibiting calcium salt scale formation in an aqueous system in aselected alkaline chemical pulping process comprising (a) determiningthe calcium salt scale inhibition profiles of phosphonate concentrationand process temperature as a function of time for phosphonatecompositions admixed with the black liquor composition recovered fromthe digester of the chemical pulping process, (b) identifying thecalcium salt scale inhibition capability required by the selectedchemical pulping process based on the process operating conditions oftime, temperature and pressure, and the black liquor composition, (c)selecting the appropriate phosphonate composition and phosphonate useconcentration to effectively inhibit calcium salt scale formation in theselected chemical pulping process when the phosphonate is admixed withthe black liquor composition recovered from the digester of the selectedalkaline chemical pulping process based on steps (a) and (b), and (d)admixing the selected phosphonate composition with the black liquorcomposition in the selected alkaline chemical pulping process during theblack liquor recovery stage of the chemical pulping process; wherein theselected phosphonate composition is as defined above for this invention.

[0097] A fourth embodiment of the invention relates to a method forinhibiting calcium salt scale formation in an aqueous system in aselected alkaline chemical pulping process comprising (a) identifyingthe calcium salt scale inhibition capability required by the selectedchemical pulping process based on the process operating conditions oftime, temperature and pressure, and the black liquor composition, (b)selecting the appropriate phosphonate composition and phosphonate useconcentration to effectively inhibit calcium salt scale formation in theselected alkaline chemical pulping process when the phosphonate isadmixed with the black liquor composition recovered from the digester inthe selected alkaline chemical pulping process based on step (a) and thecalcium salt scale inhibition profiles of phosphonate concentration andprocess temperature as a function of time for phosphonate compositionsadmixed with the black liquor composition recovered from the digester ina chemical pulping process, and (c) admixing the selected phosphonatecomposition with the black liquor composition recovered from thedigester in the selected alkaline chemical pulping process during thedigestion stage of the chemical pulping process; wherein the selectedphosphonate composition is as defined above for this invention.

[0098] In the third and fourth embodiments of the invention, the calciumsalt scale inhibition profiles of phosphonate concentration and processtemperature as a function of time for phosphonate compositions admixedwith the black liquor composition recovered from the chemical pulpingprocess digester can be determined by conducting laboratory experiments,such as described herein, or by conducting larger scale testing. As eachchemical pulping process will vary depending on the type of wood beingprocessed, the specific operating conditions used, the black liquorcomposition, the composition in the digester, and the like, the specificphosphonate or phosphonate blend and the required use concentration ofsame necessary to achieve the desired scale inhibition will be dependentupon the specific chemical pulping process. By utilizing the calciumsalt scale inhibition profiles in conjunction with the calcium saltscale inhibition capability required by the selected chemical pulpingprocess based on its process operating conditions of time, temperatureand pressure, the black liquor composition, and the aqueous digestercomposition, one of ordinary skill in the art may select the appropriatephosphonate composition and phosphonate use concentration to effectivelyinhibit calcium salt scale formation in the selected chemical pulpingprocess when the phosphonate is admixed with the black liquorcomposition in the selected chemical pulping process.

[0099] The invention is further described in the following Exampleswhich are not intended to limit or restrict the invention. Unlessotherwise indicated all quantities and percents are expressed in weight.

EXAMPLES

[0100] A calcium salt scale test of black liquor obtained from a Kraftpulp mill in the upper mid-western United States was employed in thefollowing examples which follow and illustrate the use of thecompositions of this invention in the process of this invention. Incarrying out these tests, samples were taken of a composition of theblack liquor at selected times during the test. The concentration oftotal calcium and inhibited calcium were determined analytically usingAtomic Absorption Spectroscopy (AA). Inhibited calcium is the amount ofcalcium able to pass through a 0.45 μm filter. The general proceduredescribed below was followed. Additionally, the tests were generallycarried out at the selected inhibitor level of 100 parts per million(ppm) active acid based on the amount of black liquor, for eachphosphonate composition tested, and without inhibitor present.

Black Liquor Test

[0101] The Black Liquor Test used herein was developed to gauge theperformance of calcium salt scale inhibition of compositions of thisinvention in a black liquor composition. The black liquor compositiontemperature was ramped from ambient temperature to 150° C. in about 45minutes to one hour and then maintained at 150° C. for an additional oneto two hours. Samples were taken from the condenser line of the ParrBomb Reactor using a liquid cooled extractor at various time intervalsunder pressure and temperature during the test to monitor calciumconcentrations using a procedure for determination of atomic absorptionoutlined below.

Procedure for Charging the Parr Bomb Reactor and Test Conditions

[0102] A weak black liquor sample (about 15 wt. % solids) was taken froma sample point prior to the black liquor evaporators in the Kraftprocess described above.

[0103] The charge of phosphonate employed is on an active acid basisbased upon the weight of black liquor charged to the Parr Bomb Reactor.As used herein, the level on an active acid basis is the amount (ppm) ofpure free acid that is the molar equivalent of the actual dose of thespecific phosphonate(s) used.

Preparation of Black Liquor Sample

[0104] Approximately 1.5 L of the weak black liquor obtained above wastransferred to a 2 L volumetric flask.

[0105] In the control run, no inhibitor was added to the black liquor.In the inventive runs, enough inhibitor was added to the contents of theflask to reach the desired concentration in 2 L, and weak black liquorwas added to fill to the 2 L mark.

Charge of the Parr Bomb Reactor and Monitoring of Calcium Release Test

[0106] Black liquor was prepared according to the above procedure.

[0107] Prior to running each test, the Parr Bomb Reactor was acidcleaned using a 10% sulfuric acid solution to remove any existingdeposits. After the acid cleaning, the digester was rinsed withdeionized water.

[0108] Black liquor with or without inhibitor (1.5 L) was transferred tothe Parr Bomb Reactor (2 L) and the initial temperature recorded. Theextractor line was purged with nitrogen and a 5 mL sample was taken forAA analysis. The heating sequence was initiated and time recorded ast=0. The heating sequence was to heat the contents of the Parr BombReactor from room temperature to 150° C. in 1 hour and to hold at 150°C. for the remainder of the test (approx. 1-2 hours).

[0109] (The AA analysis is done by atomic absorption by flame photometryusing a Perkin-Elmer Model 100 spectrometer; see generally, InstrumentalMethods of Analysis, Hobart H. Willard, Lynn L. Merritt, Jr.; John ADean, 4th Edition, D. Van Nostrand Company, Inc. August 1965)

[0110] Quantitatively one milliliter (mL) of the sample was transferredto a centrifuge tube with 5 mL of 4% HCl solution and AA was used todetermine the calcium content of the sample, i.e. Total Calcium. Theremaining sample was drawn into a 10 mL syringe and filtered through a0.45-μm syringe filter. Quantitatively one mL of the filtrate wastransferred to a centrifuge tube with 5 mL of 4% HCl solution and AA wasused to determine the calcium content of the filtrate, i.e. InhibitedCalcium.

[0111] Every 10 minutes for the length of the test, e.g. approximately2-3 hours, the liquor in the condenser line was purged, a temperaturemeasurement was made, and approx. a 5 mL liquor sample was pulled. TheAA analysis procedure as described above was then repeated. At the endof the test, the calcium content and temperature data were plottedversus time.

[0112] Each example below was carried out according to the generalprocedure cited above. All levels are given in parts per millionphosphonate on an active basis by weight of black liquor.

[0113] The phosphonates used in the examples were obtained from SolutiaInc. (St. Louis, Mo.). Except as specified herein, chemicals used in theexamples were obtained from Fisher Scientific.

[0114] Tables 3-7 hereinafter following provide the data for a series oftest runs performed on the black liquor using various phosphonates. Thephosphonate tested if identified by product name (as defined in Tables 1and 2 herein) in the header of each Table below. The temperature is indegrees Celsius. Parts per million (ppm) of calcium is in parts permillion by weight based on the liquor.

Example 1

[0115] A black liquor sample with no inhibitor added (Control) wastested in the test described in the Examples section. The results aregiven in Table 3 below. TABLE 3 Control—No Inhibitor Time, Minutes TotalCalcium, ppm Inhibited Calcium, ppm Temp., ° C. 0 34.4 28 19 10 34.6 2723 20 35 28.1 55 30 36.3 27.7 78 40 33.7 28.6 99 50 35.6 28.4 121 6034.1 21.7 140 75 32 4.5 151 90 30.8 2.6 150 105 31.5 1.6 150 120 29.80.6 150

Example 2

[0116] Dequest 2006 was tested in the test described in the Examplessection at 100 ppm active acid. The results are given in Table 4 below.TABLE 4 100 ppm Dequest 2006 Time, Minutes Total Calcium, ppm InhibitedCalcium, ppm Temp., ° C. 0 25 22.6 19 30 25.2 24.5 103 40 25.1 23.6 13450 26.3 22.5 150 60 26.3 22.8 150 70 26.3 22.1 150 80 26.2 19.7 150 9026.3 18.2 150 105 26.2 12.6 150 120 23.6 8.6 150

[0117] The data of this example demonstrates that a use level of 100 ppmprovided significant improvement in calcium inhibition compared to noinhibitor. The data of this example suggests that Dequest 2000 andDequest 2006 would be effective to inhibit calcium salt scale accordingto the invention.

Example 3

[0118] Dequest 2016 was tested in the test described in the Examplessection at 100 ppm active acid. The results are given in Table 5 below.TABLE 5 100 ppm Dequest 2016 Time, Minutes Total Calcium, ppm InhibitedCalcium, ppm Temp., ° C. 0 34.2 28.9 20 10 34.9 27.5 37 20 34.7 29.2 7930 36.8 27.7 120 40 36.2 28.6 148 50 37.2 26.3 150 60 35.5 25.4 149 7036 25.4 150 80 34.4 25.6 150 90 34.9 24.4 150 105 36.2 23.6 150 120 33.922.9 150

[0119] The data of this example demonstrates that a use level of 100 ppmprovided significant improvement in calcium inhibition compared to noinhibitor. The data of this example suggests that Dequest 2010 andDequest 2016 would be effective to inhibit calcium salt scale accordingto the invention.

Example 4

[0120] Dequest 2066 was tested in the test described in the Examplessection at 10 ppm and 100 ppm active acid. The results are given inTables 6 and 7 below. TABLE 6 100 ppm Dequest 2066 Time, Minutes TotalCalcium, ppm Inhibited Calcium, ppm Temp., ° C. 0 25.7 25.8 19 10 26.225.9 31 20 27.1 26.1 69 30 26.3 25.2 112 40 26.9 22.4 134 50 24.5 23 15060 26 22.7 150 70 26.9 23.1 150 80 27.1 23.5 150 90 23.5 22.5 150 10526.1 23.1 150 120 26 22.8 150

[0121] TABLE 7 10 ppm Dequest 2066 Time, Minutes Total Calcium, ppmInhibited Calcium, ppm Temp., ° C. 0 22.5 21.5 49 15 23.1 21.3 101 30 2321.5 132 40 23.5 22.1 150 50 23.3 21.2 150 60 22.9 18.5 150 70 22.9 14.4150 80 23.2 9.4 150 90 22.8 6.5 150 105 23.2 4.2 150 120 22.4 2.8 150

[0122] The data of this example demonstrates that a use level of 100 ppmprovided significant improvement in calcium inhibition compared to theuse of no inhibitor or 10 ppm inhibitor. The data of this examplesuggests that a use level of about 40 ppm to about 500 ppm for Dequest2060 and Dequest 2066 would be effective to inhibit calcium salt scaleaccording to the invention.

[0123] The preceding description is for illustration and should not betaken as limiting. Various modifications and alterations will be readilysuggested to persons skilled in the art. It is intended, therefore, thatthe foregoing be considered as exemplary only and that the scope of theinvention be ascertained from the following claims. It is furtherintended that each and every claim limitation be literally construed toinclude any and all variants which are insubstantially different fromwhat is literally recited except variants which are in the prior art.

What is claimed is:
 1. A scale inhibiting composition for inhibitingcalcium salt scale formation in alkaline aqueous mixtures of chemicalpulping processes, wherein said composition is added to the black liquorof said chemical pulping process, said composition comprising aneffective scale inhibiting amount of at least one phosphonate selectedfrom compounds having the formula:M₂O₃P—CH₂—N(R¹)—(CH₂)_(m)—N(R²)—CH₂PO₃M₂  (I), compounds having theformula: R³—C(OH)(PO₃M₂)₂  (II), compounds having the formula:N—(CH₂PO₃M₂)₃  (III), phosphonates having the formula:

amine oxides of phosphonates of formulas (I) and (III), or mixturesthereof; wherein M is independently selected from hydrogen, alkalimetal, alkaline earth metal or ammonium, R¹ and R² are independentlyselected from —CH₂PO₃M₂ or —(CH₂)_(n)—N—(CH₂PO₃M₂)₂, m is 2 or 3, n is 2or 3, and R³ is an alkyl group having 1 to 17 carbon atoms and R³ isoptionally branched and optionally unsaturated; with the provisos that:(a) said phosphonate is not a blend of a phosphonate of formula (II)with a phosphonate of formula (III), (b) said phosphonate is not a blendof a phosphonate of formula (II) with a phosphonate of the formula(M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)₂, (c) when said phosphonate isselected from phosphonates of formula (III), phosphonates of the formula(M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)₂, or phosphonates of the formula(M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)—(CH₂)₂—N—(CH₂PO₃M₂)₂, said scaleinhibiting composition does not contain a nonionic surfactant, (d) whensaid phosphonate is selected from phosphonates of formula (III), orphosphonates of the formula(M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)—(CH₂)₂—N—(CH₂PO₃M₂)₂, the amount ofsaid phosphonate on an active acid basis is greater than 25 ppm based onthe weight of black liquor recovered from the digester, and (e) when thephosphonate is selected from the phosphonates of formula (IV), theamount of the phosphonate on an active acid basis is greater than 20 ppmbased on the weight of black liquor recovered from the digester.
 2. Thecomposition of claim 1 wherein M is independently selected from hydrogenor an alkali metal.
 3. The composition of claim 2 wherein M is sodium orpotassium when M is an alkali metal.
 4. The composition of claim 1wherein R¹ and R² are CH₂PO₃M₂.
 5. The composition of claim 4 wherein mis
 2. 6. The composition of claim 1 wherein R¹ and R² are(CH₂)_(n)—N—(CH₂PO₃M₂)₂.
 7. The composition of claim 6 wherein m is 2and n is
 3. 8. The composition of claim 1 wherein R¹ is CH₂PO₃M₂ and R²is (CH₂)_(n)—N—(CH₂PO₃M₂)₂.
 9. The composition of claim 8 wherein m is 2and n is
 2. 10. The composition of claim 1 wherein R³ is an alkyl grouphaving 1 to 5 carbon atoms.
 11. The composition of claim 10 wherein R³is methyl.
 12. The composition of claim 1 wherein said phosphonate is atleast one phosphonate of formula (I).
 13. The composition of claim 1wherein said phosphonate is at least one phosphonate of formula (II).14. The composition of claim 1 wherein said phosphonate is at least onephosphonate of formula (III).
 15. The composition of claim 1 whereinsaid phosphonate is at least one phosphonate of formula (IV).
 16. Thecomposition of claim 1 wherein said phosphonate is at least one amineoxide of phosphonates of formulas (I) and (III).
 17. The composition ofclaim 1 wherein said phosphonate is a mixture of at least twophosphonates of formula (I).
 18. The composition of claim 1 wherein saidphosphonate is a mixture of at least one phosphonate of formula (I) andat least one phosphonate of formula (II).
 19. The composition of claim 1wherein said phosphonate is a mixture of at least one phosphonate offormula (I) and at least one phosphonate of formula (III).
 20. Thecomposition of claim 1 wherein said phosphonate is a mixture of at leasttwo phosphonates of formula (II).
 21. The composition of claim 1 whereinsaid phosphonate is a mixture of at least one phosphonate of formula(IV) and at least one phosphonate of formula (I), formula (II) orformula (III).
 22. The composition of claim 14 wherein said phosphonateis N(CH₂PO₃M₂)₃ and the amount of said phosphonate on an active acidbasis is about 50 ppm to about 1000 ppm based on the weight of blackliquor recovered from said digester.
 23. The composition of claim 13wherein said phosphonate is CH₃C(OH)(PO₃M₂)₂ and the amount of saidphosphonate on an active acid basis is about 20 ppm to about 200 ppmbased on the weight of black liquor recovered from said digester. 24.The composition of claim 12 wherein said phosphonate is(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂ and the amount of said phosphonate on anactive acid basis is about 10 ppm to about 1000 ppm based on the weightof black liquor recovered from said digester.
 25. The composition ofclaim 12 wherein said phosphonate is(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂ and the amount of saidphosphonate on an active acid basis is about 30 ppm to about 1000 ppmbased on the weight of black liquor recovered from said digester. 26.The composition of claim 12 wherein said phosphonate is(M₂O₃PCH₂)₂NCH₂CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂CH₂N—(CH₂PO₃M₂)₂and the amount of said phosphonate on an active acid basis is about 10ppm to about 1000 ppm based on the weight of black liquor recovered fromsaid digester.
 27. The composition of claim 15 wherein said phosphonateis

, and the amount of said phosphonate on an active acid basis is about 50ppm to about 500 ppm based on the weight of black liquor recovered fromsaid digester.
 28. The composition of claim 17 wherein said phosphonateis a mixture of:(M₂O₃PCH₂)₂NCH₂CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂CH₂—N(CH₂PO₃M₂)₂,and a second phosphonate selected from N(CH₂PO₃M₂)₃,(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂, or(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂.
 29. The composition ofclaim 28 wherein said second phosphonate is N(CH₂PO₃M₂)₃, and the amountof said mixture on an active acid basis is about 10 ppm to about 1000ppm based on the weight of black liquor recovered from said digester.30. The composition of claim 28 wherein said second phosphonate isselected from (M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂, and the amount of saidmixture on an active acid basis is about 20 ppm to about 1000 ppm basedon the weight of black liquor recovered from said digester.
 31. Thecomposition of claim 28 wherein said second phosphonate is selected from(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂, and the amount of saidmixture on an active acid basis is about 10 ppm to about 1000 ppm basedon the weight of black liquor recovered from said digester.
 32. Thecomposition of claim 17 wherein said phosphonate is a mixture of(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂ and(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂, and the amount of saidmixture on an active acid basis is about 20 ppm to about 1000 ppm basedon the weight of black liquor recovered from said digester.
 33. Thecomposition of claim 17 wherein said phosphonate is a mixture of(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂ and N(CH₂PO₃M₂)₃, and the amount of saidmixture on an active acid basis is about 30 ppm to about 1000 ppm basedon the weight of black liquor recovered from said digester.
 34. Thecomposition of claim 19 wherein said phosphonate is a mixture of(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂ and N(CH₂PO₃M₂)₃, andthe amount of said mixture on an active acid basis is about 50 ppm toabout 1000 ppm based on the weight of black liquor recovered from saiddigester.
 35. The composition of claim 18 wherein said phosphonate is amixture of(M₂O₃PCH₂)₂NCH₂CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂CH₂N—(CH₂PO₃M₂)₂and CH₃C(OH)(PO₃M₂)₂, and the amount of said mixture on an active acidbasis is about 10 ppm to about 500 ppm based on the weight of blackliquor recovered from said digester.
 36. The composition of claim 18wherein said phosphonate is a mixture of(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂ and CH₃C(OH)(PO₃M₂)₂,and the amount of said mixture on an active acid basis is about 30 ppmto about 1000 ppm based on the weight of black liquor recovered fromsaid digester.
 37. The composition of claim 1 wherein the pH of saidblack liquor is at least
 9. 38. A method for inhibiting calcium saltscale formation in alkaline chemical pulping processes comprising addingan effective scale inhibiting amount of at least one phosphonate to theblack liquor of said chemical pulping process, said compositioncomprising at least one phosphonate selected from compounds having theformula: M₂O₃P—CH₂—N(R¹)—(CH₂)_(m)—N(R²)CH₂PO₃M₂  (I), compounds havingthe formula: R³—C(OH)(PO₃M₂)₂  (II), compounds having the formula:N—(CH₂PO₃M₂)₃  (III), phosphonates having the formula:

amine oxides of phosphonates of formulas (I) and (III), or mixturesthereof; wherein M is independently selected from hydrogen, alkalimetal, alkaline earth metal or ammonium, R¹ and R² are independentlyselected from —CH₂PO₃M₂ or —(CH₂)_(n)—N—(CH₂PO₃M₂)₂, m is 2 or 3, n is 2or 3, and R³ is an alkyl group having 1 to 17 carbon atoms and R³ isoptionally branched and optionally unsaturated; with the provisos that:(a) said phosphonate is not a blend of a phosphonate of formula (II)with a phosphonate of formula (III), (b) said phosphonate is not a blendof a phosphonate of formula (II) with a phosphonate of the formula(M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)₂, (c) when said phosphonate isselected from phosphonates of formula (III), phosphonates of the formula(M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)₂, or phosphonates of the formula(M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)—(CH₂)₂—N—(CH₂PO₃M₂)₂, said scaleinhibiting composition does not contain a nonionic surfactant, (d) whensaid phosphonate is selected from phosphonates of formula (III), orphosphonates of the formula(M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)—(CH₂)₂—N—(CH₂PO₃M₂)₂, the amount ofsaid phosphonate on an active acid basis is greater than 25 ppm based onthe weight of black liquor recovered from the digester, and (e) when thephosphonate is selected from the phosphonates of formula (IV), theamount of the phosphonate on an active acid basis is greater than 20 ppmbased on the weight of black liquor recovered from the digester.
 39. Themethod of claim 38 wherein M is sodium or potassium when M is an alkalimetal.
 40. The method of claim 38 wherein R¹ and R² are CH₂PO₃M₂. 41.The method of claim 40 wherein m is
 2. 42. The method of claim 38wherein R¹ and R² are (CH₂)_(n)—N—(CH₂PO₃M₂)₂.
 43. The method of claim42 wherein m is 2 and n is
 3. 44. The method of claim 38 wherein R¹ isCH₂PO₃M₂ and R² is (CH₂)_(n)—N—(CH₂PO₃M₂)₂.
 45. The method of claim 44wherein m is 2 and n is
 2. 46. The method of claim 38 wherein R³ is analkyl group having 1 to 5 carbon atoms.
 47. The method of claim 38wherein R³ is methyl.
 48. The method of claim 38 wherein saidphosphonate is at least one phosphonate of formula (I).
 49. The methodof claim 38 wherein said phosphonate is at least one phosphonate offormula (II).
 50. The method of claim 38 wherein said phosphonate is atleast one phosphonate of formula (III).
 51. The method of claim 38wherein said phosphonate is at least one phosphonate of formula (IV).52. The method of claim 38 wherein said phosphonate is at least oneamine oxide of phosphonates of formulas (I) and (III).
 53. The method ofclaim 38 wherein said phosphonate is a mixture of at least twophosphonates of formula (I).
 54. The method of claim 38 wherein saidphosphonate is a mixture of at least one phosphonate of formula (I) andat least one phosphonate of formula (II).
 55. The method of claim 38wherein said phosphonate is a mixture of at least one phosphonate offormula (I) and at least one phosphonate of formula (III).
 56. Themethod of claim 38 wherein said phosphonate is a mixture of at least twophosphonates of formula (II).
 57. The method of claim 38 wherein saidphosphonate is a mixture of at least one phosphonate of formula (IV) andat least one phosphonate of formula (I), formula (II) or formula (III).58. The method of claim 50 wherein said phosphonate is N(CH₂PO₃M₂)₃ andthe amount of said phosphonate on an active acid basis is about 50 ppmto about 1000 ppm based on the weight of black liquor recovered fromsaid digester.
 59. The method of claim 49 wherein said phosphonate isCH₃C(OH)(PO₃M₂)₂ and the amount of said phosphonate on an active acidbasis is about 20 ppm to about 200 ppm based on the weight of blackliquor recovered from said digester.
 60. The method of claim 48 whereinsaid phosphonate is (M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂ and the amount ofsaid phosphonate on an active acid basis is about 10 ppm to about 1000ppm based on the weight of black liquor recovered from said digester.61. The method of claim 48 wherein said phosphonate is(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂ and the amount of saidphosphonate on an active acid basis is about 30 ppm to about 1000 ppmbased on the weight of black liquor recovered from said digester. 62.The method of claim 48 wherein said phosphonate is(M₂O₃PCH₂)₂NCH₂CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂CH₂N—(CH₂PO₃M₂)₂and the amount of said phosphonate on an active acid basis is about 10ppm to about 1000 ppm based on the weight of black liquor recovered fromsaid digester.
 63. The method of claim 53 wherein said phosphonate is amixture of: (M₂O₃PCH₂)₂NCH₂CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂CH₂—N(CH₂PO₃M₂)₂, and a secondphosphonate selected from N(CH₂PO₃M₂)₃, (M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂,or (M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂.
 64. The method ofclaim 63 wherein said second phosphonate is N(CH₂PO₃M₂)₃, and the amountof said mixture on an active acid basis is about 10 ppm to about 1000ppm based on the weight of black liquor recovered from said digester.65. The method of claim 63 wherein said second phosphonate is(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂, and the amount of said mixture on anactive acid basis is about 20 ppm to about 1000 ppm based on the weightof black liquor recovered from said digester.
 66. The method of claim 63wherein said second phosphonate is(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂, and the amount of saidmixture on an active acid basis is about 10 ppm to about 1000 ppm basedon the weight of black liquor recovered from said digester.
 67. Themethod of claim 53 wherein said phosphonate is a mixture of(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂ and(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂, and the amount of saidmixture on an active acid basis is about 20 ppm to about 1000 ppm basedon the weight of black liquor recovered from said digester.
 68. Themethod of claim 53 wherein said phosphonate is a mixture of(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂ and N(CH₂PO₃M₂)₃, and the amount of saidmixture on an active acid basis is about 30 ppm to about 1000 ppm basedon the weight of black liquor recovered from said digester.
 69. Themethod of claim 55 wherein said phosphonate is a mixture of(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂ and a second phosphonateselected from N(CH₂PO₃M₂)₃, and the amount of said mixture on an activeacid basis is about 50 ppm to about 1000 ppm based on the weight ofblack liquor recovered from said digester.
 70. The method of claim 54wherein said phosphonate is a mixture of(M₂O₃PCH₂)₂NCH₂CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂CH₂N—(CH₂PO₃M₂)₂and CH₃C(OH)(PO₃M₂)₂, and the amount of said mixture on an active acidbasis is about 10 ppm to about 500 ppm based on the weight of blackliquor recovered from said digester.
 71. The method of claim 54 whereinsaid phosphonate is a mixture of(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂ and CH₃C(OH)(PO₃M₂)₂,and the amount of said mixture on an active acid basis is about 30 ppmto about 1000 ppm based on the weight of black liquor recovered fromsaid digester.
 72. The method of claim 51 wherein said phosphonate is

, and the amount of said phosphonate on an active acid basis is about 50ppm to about 500 ppm based on the weight of black liquor recovered fromsaid digester.
 73. The method of claim 38 wherein said chemical pulpingprocess is a Kraft process.
 74. The method of claim 73 wherein calciumsalt scale is inhibited in the digester.
 75. The method of claim 73wherein calcium salt scale is inhibited in the brown stock washing area.76. The method of claim 73 wherein calcium salt scale is inhibited inthe black liquor recovery area.
 77. The method of claim 38 wherein saidcalcium salt is calcium carbonate or calcium sulfate.
 78. The method ofclaim 77 wherein said calcium salt is calcium carbonate.
 79. The methodof claim 38 wherein the pH of said black liquor is at least
 9. 80. Amethod for inhibiting calcium salt scale formation in an aqueous systemin a selected alkaline chemical pulping process comprising: (a)determining the calcium salt scale inhibition profiles of phosphonateconcentration and process temperature as a function of time forphosphonate compositions admixed with the black liquor compositionrecovered from the digester of said chemical pulping process, (b)identifying the calcium salt scale inhibition capability required bysaid selected chemical pulping process based on the process operatingconditions of time, temperature and pressure, and the black liquorcomposition, (c) selecting the appropriate phosphonate composition andphosphonate use concentration to effectively inhibit calcium salt scaleformation in said selected chemical pulping process when saidphosphonate is admixed with the black liquor composition recovered fromthe digester of said selected alkaline chemical pulping process based onsteps (a) and (b), and (d) admixing the selected phosphonate compositionwith the black liquor composition in said selected alkaline chemicalpulping process during the black liquor recovery stage of the chemicalpulping process; wherein said selected phosphonate composition comprisesat least one phosphonate selected from compounds having the formula:M₂O₃P—CH₂—N(R¹)—(CH₂)_(m)—N(R²)—CH₂PO₃M₂  (I), compounds having theformula: R³—C(OH)(PO₃M₂)₂  (II), compounds having the formula:N—(CH₂PO₃M₂)₃  (III), phosphonates having the formula:

amine oxides of phosphonates of formulas (I) and (III), or mixturesthereof; wherein M is independently selected from hydrogen, alkalimetal, alkaline earth metal or ammonium, R¹ and R² are independentlyselected from —CH₂PO₃M₂ or —(CH₂)_(n)—N—(CH₂PO₃M₂)₂, m is 2 or 3, n is 2or 3, and R³ is an alkyl group having 1 to 17 carbon atoms and R³ isoptionally branched and optionally unsaturated; with the provisos that:(i) said phosphonate is not a blend of a phosphonate of formula (II)with a phosphonate of formula (III), (ii) said phosphonate is not ablend of a phosphonate of formula (II) with a phosphonate of the formula(M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)₂, (iii) when said phosphonate isselected from phosphonates of formula (III), phosphonates of the formula(M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)₂, or phosphonates of the formula(M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)—(CH₂)₂—N—(CH₂PO₃M₂)₂, said scaleinhibiting composition does not contain a nonionic surfactant, (iv) whensaid phosphonate is selected from phosphonates of formula (III),phosphonates of the formula (M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)₂, orphosphonates of the formula(M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)—(CH₂)₂—N—(CH₂PO₃M₂)₂, the amount ofsaid phosphonate on an active acid basis is greater than 25 ppm based onthe weight of black liquor recovered from the digester, and (v) when thephosphonate is selected from the phosphonates of formula (IV), theamount of the phosphonate on an active acid basis is greater than 20 ppmbased on the weight of black liquor recovered from the digester.
 81. Amethod for inhibiting calcium salt scale formation in an aqueous systemin a selected alkaline chemical pulping process comprising: (a)identifying the calcium salt scale inhibition capability required bysaid selected chemical pulping process based on the process operatingconditions of time, temperature and pressure, and the black liquorcomposition, (b) selecting the appropriate phosphonate composition andphosphonate use concentration to effectively inhibit calcium salt scaleformation in said selected alkaline chemical pulping process when saidphosphonate is admixed with the black liquor composition recovered fromthe digester in said selected alkaline chemical pulping process based onstep (a) and the calcium salt scale inhibition profiles of phosphonateconcentration and process temperature as a function of time forphosphonate compositions admixed with the black liquor compositionrecovered from the digester in a chemical pulping process, and (c)admixing the selected phosphonate composition with the black liquorcomposition recovered from the digester in said selected alkalinechemical pulping process during the digestion stage of the chemicalpulping process; wherein said selected phosphonate composition comprisesat least one phosphonate selected from compounds having the formula:M₂O₃P—CH₂—N(R¹)—(CH₂)_(m)—N(R²)—CH₂PO₃M₂  (I), compounds having theformula: R³—C(OH)(PO₃M₂)₂  (II), compounds having the formula:N—(CH₂PO₃M₂)₃  (III), phosphonates having the formula:

amine oxides of phosphonates of formulas (I) and (III), or mixturesthereof; wherein M is independently selected from hydrogen, alkalimetal, alkaline earth metal or ammonium, R¹ and R² are independentlyselected from —CH₂PO₃M₂ or —(CH₂)_(n)—N—(CH₂PO₃M₂)₂, m is 2 or 3, n is 2or 3, and R³ is an alkyl group having 1 to 17 carbon atoms and R³ isoptionally branched and optionally unsaturated; with the provisos that:(i) said phosphonate is not a blend of a phosphonate of formula (II)with a phosphonate of formula (III), (ii) said phosphonate is not ablend of a phosphonate of formula (II) with a phosphonate of the formula(M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)₂, (iii) when said phosphonate isselected from phosphonates of formula (III), phosphonates of the formula(M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)₂, or phosphonates of the formula(M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)—(CH₂)₂—N—(CH₂PO₃M₂)₂, said scaleinhibiting composition does not contain a nonionic surfactant, (iv) whensaid phosphonate is selected from phosphonates of formula (III),phosphonates of the formula (M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)₂, orphosphonates of the formula(M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)—(CH₂)₂—N—(CH₂PO₃M₂)₂, the amount ofsaid phosphonate on an active acid basis is greater than 25 ppm based onthe weight of black liquor recovered from the digester, and (v) when thephosphonate is selected from the phosphonates of formula (IV), theamount of the phosphonate on an active acid basis is greater than 20 ppmbased on the weight of black liquor recovered from the digester.